High speed machining of metals has generated great technological interest because it has the potential to offer excellent surface finish under dry machining conditions, increase productivity and decrease the cost. In finish turning operations, high speed coupled with low feed rates have been used successfully to achieve excellent surface finish so that a subsequent finish grinding operation is eliminated, resulting in substantial cost savings. Polycrystalline cubic boron nitride (PCBN) tools have been found to be the tools of choice for high speed machining because they exhibit diamond-like structure, high hardness and good thermal conductivity. For example, PCBN tools are successfully used at high speeds (>7,200 feet (2,194 m) per minute) and low feed rates (0.006″ (0.15 mm) to achieve the excellent surface finish (Ra<1 micrometer) as required, for example, in cast iron brake rotors. The problem, however, which has plagued the growth of high speed machining of cast iron, is chemical wear of the tool occurring at the cutting edge caused by unknown variables in pearlitic iron castings which cause unpredictable tool life and poor surface finish.
My earlier patent (U.S. Pat. No. 6,537,395) directed to processes for producing gray cast iron was designed for improving tool life and surface finish in high speed machining with PCBN. That invention is based on the beneficial effects of engineering microalloying additions with strong affinity for nitrogen, carbon and oxygen in the work-piece materials in order to suppress dynamic strain aging during machining just-in-time (JIT) castings and to protect the PCBN cutting edge against oxidation by in-situ formation of chemically stable refractory oxides. The cast iron workpiece design with microalloying additions for improving tool life and surface finish in high speed machining with PCBN and silicon nitride tools is the subject of my related patent (U.S. Pat. No. 6,395,107 B1). The current technological trend in automotive companies is to outsource castings on a global basis. Further, compacted graphite cast iron has emerged just recently as a material of choice in the development of new engines that can withstand high firing pressure. Accelerated chemical tool wear at high cutting speeds, however, has proven to be a significant obstacle in the way of achieving the required line speeds for high productivity in automotive manufacturing of compacted graphite iron engine blocks. Thus, there is a significant incentive to develop additional solutions to combat chemical tool wear, which are based on a quantitative understanding of the mechanisms underlying accelerated chemical wear in high speed machining of metals.
The present invention is aimed at improving the tool life and surface finish while achieving improved productivity using high cutting speeds. Typical automotive applications use high cutting speeds of 5 to 35 m/s and low feeds of 0.05 to 0.15 mm, where the tribology at the tool-chip contact involves atomic contact or seizure and the chips exhibit shear localized or segmented morphology. In contrast, the prior art on vibration assisted machining (VAM) is confined to precision machining at very low cutting speeds of a few centimeters per second and exceedingly low feeds of a few micrometers; whereby excellent surface finish is obtained at very low productivity, and where the chip morphology is of the continuous flow type and the tribology at the tool-chip contact is sliding, involving asperity contact. Under these conditions, accelerated chemical tool wear is not an issue. But in high speed machining, accelerated chemical tool wear is the root cause of poor tool life. As the cutting speed is increased, the tribology of sliding occurring at low cutting speed changes over to seizure, involving atomic contact at the tool-chip interface at high cutting speeds. The chip morphology changes over from the flow type at low cutting speeds to shear localized chip with nanocrystalline grain formation in the shear localized region at high cutting speeds. In consequence, accelerated chemical wear is caused by rapid dissolution of tool into chip, by nanocrystalline grain boundary diffusion. In addition, the oxidation of the tool occurs when the cracks associated with segmentation of the chip exposes the tool to an oxidizing environment. According to the present invention, the tool is vibrated at a frequency exceeding the critical frequency of shear localization or segmentation of chip, characteristic of the dynamic behavior of the workpiece material under cutting conditions in order to suppress the formation of shear localized or segmented chip morphology, thereby preventing accelerated chemical tool wear.
Chemical wear is the dominant mechanism of tool failure in high speed machining. In the case of hardened steel, I have discovered that nanocrystalline grains form in the interfacial layer at the tool-chip contact, which causes accelerated dissolution of tool material into the chip or workpiece at the interface therebetween. In the case of cast iron with low strain to fracture compared to steel, oxidation of the tool can result in tool wear, which is caused by oxygen ingress through cracks associated with segmented chip morphology. According to the present invention, vibration of the tool is shown to be beneficial for prolonging the tool life under these conditions as well, provided the frequency of vibration of the tool is above the critical frequency of shear localization occurring in the primary shear zone of the chip as in the case of hardened steel or segmentation of the chip as in the case of cast iron. It is another object of this invention to provide methods which suppress the mechanisms that cause accelerated chemical tool wear.
A further object of this invention to provide processes which minimize chemical wear at the cutting edge of tools during high speed machining of metals.
Yet another object of this invention is to provide physical techniques rather than chemical composition modifications to prolong tool life.